The present invention relates to an electromagnetic valve used for correcting the difference between the angle of a steering wheel and that of a steered wheel in the hydraulic circuit of a vehicle hydraulic power steering device and to a hydraulic power steering device using such a valve.
Recently, many types of industrial vehicles, including forklifts, have been equipped with power steering devices, some of which include hydraulic power steering devices. However, the hydraulic power steering devices have drawbacks. In a steering valve, which forms part of the hydraulic power steering device and is driven by the manipulation of a steering wheel, the relationship (discharge efficiency=actual discharge amount/theoretical discharge amount) between the movement of the steering wheel and the amount of oil discharged from the steering valve changes slightly depending on the angular velocity of the steering wheel. Also, oil can not be prevented from leaking slightly from the oil hydraulic circuit. For this reason, a difference in position between the steering wheel and the steered wheel occurs.
In order to solve this problem, Japanese Patent Publication No. 3-30544, Japanese Patent Publication No. 4-24270 and Japanese Utility Model Publication No. 7-5364 disclose devices for correcting the difference in position between the steering wheel and the steered wheel. FIG. 5 shows such a device for correcting the angle of a steering wheel.
An entire-hydraulic power steering device 71 includes a steering valve 73 driven by manipulation of a steering wheel 72 and a steering cylinder 75 for steering the tires, or steered wheels 74. The steering valve 73 and the steering cylinder 75 are connected by two hydraulic pipes 76, 77. An electromagnetic valve (a correcting valve) 78 is located in the pipes 76, 77 at an intermediate location of the pipes 76, 77. When the steering wheel 72 is manipulated, the steering valve 73 discharges hydraulic oil flowing in from a hydraulic pump 79 to the pipes 76, 77 corresponding to the manipulation direction of the steering wheel and exhausts return oil (returning from the steering cylinder 75) from the other of the pipes 76, 77 to an oil tank 80.
A controller 81 is connected to a first sensor 82 for detecting the angular position of the steering wheel and a second sensor 83 for detecting the steered angle of one of the steered wheels 74. The controller 81 calculates a target angle of the steered wheel 74 based on a value detected by the first sensor and compares the actual angle of the steered wheel 74 detected by the second sensor with the target angle to determine the difference between the target angle and the actual angle. When the difference exceeds a tolerance limit, the controller 81 causes the electromagnetic valve 78 to open. When the steering wheel 72 is manipulated while the valve 78 is open, some of the hydraulic oil is returned to the oil tank 80 through the valve 78, which decreases the amount of hydraulic oil sent to the steering cylinder 75. Accordingly, the steering wheel 72 races. When the angle of the steering wheel coincides with the angle of the wheel 74, the valve 78 is closed. This procedure corrects the relationship between the wheel 74 and the steering wheel 72.
The electromagnetic valve 78 in the prior art has a structure shown in FIG. 6. The valve 78 includes a driving control unit 85 and a manifold 86. The driving control unit 85 includes a solenoid 87 and a plunger (valve body) 88. The plunger 88 is urged downwardly and away from a plug 90 by a spring 89 and is driven upwardly in a direction approaching the plug 90 by the solenoid 87, thus the plunger 88 is displaced axially. The plunger 88 has a ball 92 fitted on the tip portion thereof. A spool 91 is located below the plunger 88. Oil paths 91a, 91b are formed in the spool 91 to return the hydraulic oil. Also, an oil path 91c is formed by a recess in the outer peripheral surface of the spool 91. When the hydraulic oil is not returned, the hydraulic oil passes through the oil path 91c. By vertically displacing the plunger 88, the ball 92 is separated from and abutted against the upper end surface of the spool 91, which opens and closes the return oil paths 91a, 91b. Four straight-type fittings 93a, 93b, 94a, 94b are attached to the manifold 86. The left fittings 93a and 93b are respectively connected to the two pipes extending from the steering valve 73 and the right fittings 94a and 94b are respectively connected to the two pipes extending from the steering cylinder 75. A passage 95 connects the fittings 93a and 94a at the upper side of the manifold 86, and a passage 96 connects the fittings 93b and 94b at the lower side of the manifold 86. The terms "upper" and "lower" refer to the orientation of FIG. 6.
When the valve 78 is closed (in the state shown in FIG. 6), the feed oil and the return oil pass through the passages 95 and 96. In FIG. 6, the passage 95 at the upper side of the manifold 86 includes the oil path 91c. Since the oil path 91c is formed in the outer peripheral surface of the spool 91, it is difficult to make the oil path 91c with a large inside diameter. Therefore, the oil path 91c has a relatively small inside diameter.
Thus, even if the valve 78 is closed, either the feed oil or the return oil must pass through the narrow oil path 91c around the spool 91. Since the oil path 91c is not only narrow, but it bends, the hydraulic oil passing through this path is subject to a pressure loss. Accordingly, since the pressure of the hydraulic oil sent to the steering cylinder 75 is relatively weak, this adversely affects the response to manipulation of the steering wheel 72. The consequences are, for example, that the response of the steered wheel 74 to the manipulation of the steering wheel 72 is delayed, and the steered wheel 74 cannot be turned if the steering wheel 72 is only slightly manipulated. Therefore, drivers feel that the steered wheel 74 has a poor response to the manipulation of the steering wheel 72.
Also, when hydraulic oil passes through the narrow oil path 91c, friction heat is likely to occur, which will raise the temperature of the oil. Warmer oil causes heat damage to packing materials and leads to oil leakage. Further, warmer oil imposes a heat burden on parts of the electric system, such as the solenoid 87, which shortens the life of the electric system.